1. Field of the Invention
The invention relates to a method for coordinate measurement on workpieces using a coordinate measuring machine (CMM) in which the measured data are corrected by means of stored correction values.
2. Relevant Prior Art
To increase the measuring accuracy of CMMs, the measured data, i.e. the coordinate values supplied by the measuring scales in the machines axes have been mathematically corrected for some time now. Normally, corrections are made as a result of the static guideway deviations of the CMM measuring slides from a straight and orthogonal coordinate system, these deviations being derived according to the so-called "rigid model". Such correction methods are described, for example in the paper "Numerical Error Correction of a Coordinate Measuring Machine" by K. Busch, H. Kunzmann and F. Waldele in the Proceedings of the International Symposium on Metrology for Quality Control in Production, Tokyo 1984, pp. 284-288, and in U.S. Pat. No. 4,819,195.
For tactile coordinate measuring machines, it is also a known method to determine the amount of probe bending during contact with the workpiece being measured and to take this amount into account in the subsequent data evaluation. Such a method is described, for example, in the paper "Korrektur der Taststiftbiegung bei Messung mit Mehrkoordinaten-Messgeraten" (Correction of Probe Bending When Measuring with Coordinate Measuring Machines) by A. Weckenmann, G. Goch and H. D. Springborn, in Feinwerktechnik und Messtechnik 87 (1979) 1, pp. 5-9 and in the paper "Messende Taster mit mehreren Freiheitsgraden" (Measuring Probes with Several Degrees of Freedom) by W. Lotze in Technische Rundschau, issue 50 (1992) pp. 20-25.
In this method, the bending tensor of the respective probe is defined by probing a calibration sphere several times using different measuring forces. From this, the correction values for the different probes used for measuring a workpiece are calculated and stored, and included in the data evaluation of measurements of unknown workpieces.
Furthermore, it is a known method to correct the dynamic deviations of the measuring slideway of a CMM, i.e., the errors which occur due to the fact that the CMM's probe head mounted on the end of the horizontal arm is not at rest, but vibrates about its rest position as a result of error influences such as drive acceleration, for example. Such a correction method is described, for example, in U.S. Pat. No. 4,333,238. Such dynamic correction methods use sensors which are mounted in the vicinity of the vibrating probe head and which generate signals representing the course of the spurious oscillation at the moment the probing contact is made.
However, the described known methods all correct only a part of the errors which are likely to occur when measuring with a CMM. The measuring uncertainty of CMMs can be reduced with these methods, but considerable residual errors still remain which limit the measuring accuracy of the CMM.
In German Patent Application P 44 18 550.2, corresponding to a U.S. patent application Ser. No. 08/452,803, which is filed contemporaneous herewith, and which is incorporated herein by reference, a method is described which takes into account for the first time flexibility and elastic behavior of the structure and the measuring slides of the CMM, that is, their flexibility due to the forces applied during probing by the workpiece on the CMM probe head. Although these forces are relatively low and although the distortion of the structure of the machine caused by them is only low, they may still appear as measuring errors if the specified measuring uncertainty of the CMM is in the submicron range. This distortion depends on the extension of the CMM measuring slide and therefore causes deviations of the measured coordinate data of different magnitude as a function of the position of the probe head within the CMM measuring range.
However, the probing force is only one of several forces acting on the coordinate measuring machine during the probing process. Other forces are caused by bearing stress due to acceleration of the measuring slides and inertia of masses in the moving parts, which also lead to distortion of the CMM structure and thus to measuring errors.